1. Field of the Invention
The present invention relates generally to a time-period of a signal with that of a predetermined reference signal and, more particularly, to a time-period comparing device adapted to computer systems and electronic switching systems. The device may be used, for example, for the monitoring for cooling fans for cooling the electrical components of the systems.
In a computer system or an electronic switching system, a considerable number of electrical components, such as integrated circuits (hereinafter referred to as ICs) are contained in a cabinet which is of a relatively small size. During the operation of the components, they generate heat. However, the components, including transistors, diodes, etc., are designed to function only under specified conditions. For example, the components operate properly only within a predetermined range of temperature. Furthermore, the components may be damaged if exposed to temperatures which are too high. At present, in order to prevent the components from being overheated, the components are cooled by using cooling fans. Therefore, it is necessary to monitor the rotational velocity of the fans continuously and to provide an alarm signal when the rotational velocity drops below a predetermined value.
2. Description of the Prior Art
In one prior art device for monitoring the rotational velocity of a fan, a motor for driving the fan is provided with a tachometer generator. Thus the device produces an alarm signal when the output of the tachometer generator drops below a predetermined value. However, it is difficult to mount the device on a cooling fan system since the device is of a relatively large size. Furthermore, the cost of the device is high.
Another prior art device for monitoring the rotational velocity of a fan includes a sensor for detecting the rotational velocity of the fan photo-electrically or electromagnetically. This monitoring device also includes an electric circuit for comparing the rotational velocity signal generated by the sensor with a reference signal which is generated by using a monostable multivibrator. However, since the time-period of the rotational velocity of the fan is relatively long, a high capacitance capacitor for determining the reference time-period of the monostable multivibrator is necessary. Therefore, if this device is manufactured as an IC, the capacitor has to be connected outside the IC so that the number of terminals of the IC increases. This terminal number is limited by mechanical tolerance associated with the physical dimensions of the IC. In addition, elements, such as capacitors and resistors, for determining the time-period of the reference signal, are affected by various ambient conditions, for example, ambient temperature, so that the time-period of the reference signal is unstable.